Interferent and baseline drift correcting sensor system

ABSTRACT

A sensor system that removes responses from an interferent and/or corrects for baseline drift of a sensor to determine a presence, a concentration or a change in concentration of a target material in a gaseous environment. Fluid flowing into the system may be directed by a valve arrangement to either a first fluid flow path or a second fluid flow path. The target material may be absorbed by a filter material in the first fluid flow path. Fluid flowing along the second gas flow path passes directly to the sensor. Responses of the sensor to fluids from the first and second fluid flow paths may be used to determine a presence, concentration or change in concentration of the target material.

BACKGROUND

Embodiments of the present disclosure relate to apparatus and methods for sensing a target gas in an environment. More particularly, but not by way of limitation, a gas sensor system is provided that corrects/removes effects of interferent and/or corrects baseline drift. Some embodiments of the present disclosure relate to apparatus and methods for sensing a polar gas, e.g. 1-methylcyclopropene (1-MCP).

Thin film transistors (TFTs) have been previously used as gas sensors. For example, such use of thin film transistors as gas sensors is described in Feng et al, “Unencapsulated Air-stable Organic Field Effect Transistor by All Solution Processes for Low Power Vapor Sensing” Scientific Reports 6:20671 DOI: 10.1038/srep20671 and Besar et al, “Printable ammonia sensor based on organic field effect transistor”, Organic Electronics, Volume 15, Issue 11, November 2014, Pages 3221-3230. In thin film transistor gas sensors, a semiconducting layer is in electrical contact with source and drain electrodes and a gate dielectric is disposed between the semiconducting layer and a gate electrode. Interaction of a target material with the TFT gas sensor may alter the drain current of the TFT gas sensor.

Ethylene produced by plants can accelerate ripening of climacteric fruit, the opening of flowers, and the shedding of plant leaves. 1-methylcyclopropene (1-MCP) is known for use in inhibiting such processes.

It may be desirable to determine the presence and/or concentration of certain materials in a gaseous environment. However, a gas sensor used for this purpose may respond to one or more materials in the environment other than the target material; the concentration of background materials in the environment that the gas sensor responds to may change over time; or the response of the gas sensor to a target or background material may change as the gas sensor ages.

SUMMARY

In some embodiments there is provided apparatus configured to determine a presence, a concentration or a change in concentration of a target material in an environment, e.g. a liquid or gaseous environment. The apparatus may contain a sensor, e.g. a gas sensor, configured to respond to the target material. The apparatus may contain a fluid inlet, e.g. a liquid or gas inlet, configured to draw liquid or gas from an environment into the apparatus. The apparatus may contain a valve arrangement configured to direct fluid drawn from the environment to a first fluid flow path in fluid communication with the sensor or to a second fluid flow path in fluid communication with the gas sensor.

Optionally, the first fluid flow path is disposed between the gas sensor and a first valve of the valve assembly and the second fluid flow path is disposed between the gas sensor and a second valve of the valve assembly.

Optionally, the first and second fluid flow paths are disposed between the sensor and a three-way valve of the valve assembly.

Optionally, the apparatus further has a third fluid flow path in fluid communication with the sensor and disposed between a valve of the valve arrangement and the sensor.

Optionally, in the case where the fluid is a gas, a humidification stage, e.g. comprising or consisting of a water reservoir, in fluid communication with the first fluid flow path is disposed in fluid communication with the first fluid flow path between the valve assembly and the sensor. Optionally, the humidification stage, e.g. the water reservoir, is in fluid communication with the first and second fluid flow paths. Optionally, a saturated salt solution is disposed in the water reservoir. A desiccant may be disposed in a dehumidification stage for dehumidification of gas drawn into the apparatus, the dehydrated gas being subsequently rehydrated at the humidification stage. Optionally, the dehumidification stage is disposed between the inlet and the valve assembly. Optionally, the desiccant is disposed in the second fluid flow path.

Optionally, the sensor is a thin film transistor.

In some embodiments there is provided a method of determining a presence, concentration or change in concentration of a target material in an environment, e.g. a liquid or gaseous environment. The process may include measuring, in either order, a first response and a second response of a sensor of the sensor apparatus. The first response may be a response of the sensor to gas from the first flow path which has been treated to remove the target material from the fluid. The second response may be a response of the sensor to gas from the second flow path which has not been treated to remove the target material from the fluid. The method may include subtracting the first sensor measurement or a derivative thereof from the second sensor measurement or a derivative thereof.

Optionally, a filter material for removing the target material from the fluid is disposed in the first fluid flow path.

In some embodiments, the filter material is a molecular sieve.

In some embodiments, the filter material is a desiccant filter material, optionally silica gel.

Optionally, the first sensor measurement is generated by exposing the sensor to a sample from the environment which has been exposed to the desiccant filter material and rehydrated.

Optionally, the system is a gas sensor system, the sensor is a gas sensor and the environment from which the fluid is drawn is a gaseous environment.

Optionally, the target material is an alkene, optionally ethylene or 1-methylcyclopropene.

Optionally, the first and second sensor measurements or derivatives thereof are a selected from a change in resistance of the sensor; or, in the case of a TFT sensor, a change in drain current of the sensor; and a change in dI/dt wherein I is drain current and t is time.

In some embodiments there is provided a method of withdrawing 1-methylcyclopropene from a gaseous environment in which the environment is contacted with silica gel.

In some embodiments there is provided method of determining a presence, a concentration or a change in concentration of a target material, e.g. a target gas, in an environment. The method may include measuring a rate of change in drain current or resistance of a TFT gas sensor upon exposure to the environment for a time period. The method may include determining, from the rate of change, a presence, a concentration of a change in concentration of the target material at a point during the time period.

Optionally, the method includes determining dI/dt for the time period wherein I is drain current and t is time; identifying dI/dt peaks; and determining, from the dI/dt peaks, a presence, a concentration or a change in concentration of the target material at the point during the time period. Optionally, the gas is 1-methylcyclopropene.

In some embodiments, the present disclosure provides a method of determining a presence, concentration or change in concentration of a target material in a gas, the method comprising:

dehumidifying the gas;

rehumidifying the dehumidified gas; and

measuring a response of a gas sensor configured to detect the target material.

Optionally, the gas is rehumidified to within a predetermined range.

Optionally, the gas is dehumidified by bringing it into contact with a molecular sieve or a solid metal salt.

Optionally, the gas is rehumidified by a saturated salt solution.

Optionally, the target gas is an alkene.

Optionally, the target gas is 1-methylcyclopropene or ethylene.

According to some embodiments, the present disclosure provides a gas sensor apparatus configured to determine a presence, a concentration or a change in concentration of a target material in a gaseous environment, comprising:

a dehumidification stage configured to dehumidify gas drawn into the apparatus from an inlet;

a gas sensor; and

a rehumidification stage disposed between and in fluid communication with the dehumidification stage and the gas sensor, the rehumidification stage being configured to rehumidify the dehumidified gas.

DESCRIPTION OF THE DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology.

FIG. 1 illustrates gas sensor apparatus according to some embodiments having first and second gas flow paths in which a target material is removed and is not removed, respectively, from gas entering the apparatus before reaching a gas sensor;

FIG. 2 illustrates a process according to some embodiments for determining the presence, concentration or change in concentration of a target gas in an environment using measurements of a gas sensor apparatus in a first and second state;

FIG. 3 illustrates a process according to some embodiments for determining the presence, concentration or change in concentration of a target gas in an environment using measurements of a gas sensor apparatus in a first state, a second state and a closed state;

FIG. 4 illustrates gas sensor apparatus according to some embodiments in which desiccated gas is rehydrated before reaching a gas sensor;

FIG. 5 illustrates gas sensor apparatus according to some embodiments in which desiccated and untreated gas are both exposed to water before reaching a gas sensor;

FIG. 6 illustrates gas sensor apparatus according to some embodiments in which gas entering the apparatus is dehumidified before being separated into first and second gas flow paths and subsequently rehumidified before reaching a gas sensor;

FIG. 7 illustrates gas sensor apparatus according to some embodiments in which gas entering the apparatus is separated into first and second gas flow paths in which gas in one or both gas flow paths is dehumidified and subsequently rehumidified before reaching a gas sensor;

FIG. 8 illustrate gas sensor apparatus according to some embodiments in which gas entering the apparatus is dehumidified and subsequently rehumidified;

FIG. 9 illustrates a bottom gate, bottom contact TFT sensor according to some embodiments;

FIG. 10 illustrates a bottom gate, top contact TFT sensor according to some embodiments;

FIG. 11 illustrates a top gate, bottom contact TFT sensor according to some embodiments;

FIG. 12 illustrates a top gate, top contact TFT sensor according to some embodiments; and

FIG. 13A is a graph of average resistance change vs. time for OTFT gas sensor for which background has been subtracted according to some embodiments;

FIG. 13B is a graph of average resistance change vs theoretical (assumed) 1-MCP concentration for the data of FIG. 13A;

FIG. 14 is a graph of average resistance change vs theoretical 1-MCP concentration after 5 and 20 minutes exposure of an OTFT gas sensor, for which background has been subtracted according to some embodiments;

FIG. 15A is a graph of current vs. time upon exposure to 1-MCP of multiple OTFT gas sensors having the same structure but formed in different batches;

FIG. 15B is a graph of response of the gas sensors of FIG. 15A vs. 1-MCP concentration;

FIG. 16A is a graph of dI/dt vs time upon exposure to 1-MCP of multiple OTFT gas sensors having the same structure but formed in different batches;

FIG. 16B is a graph of dI/dt vs. 1-MCP concentration for the of the gas sensors of FIG. 16A;

FIG. 17A is a graph of humidity of an output gas following humidification only of an input gas; and

FIG. 17B is a graph of humidity of an output gas following dehumidification and rehumidification of an input gas.

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION

Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

As used herein, by a material “over” a layer is meant that the material is in direct contact with the layer or is spaced apart therefrom by one or more intervening layers.

As used herein, by a material “on” a layer is meant that the material is in direct contact with that layer.

A layer “between” two other layers as described herein may be in direct contact with each of the two layers it is between or may be spaced apart from one or both of the two other layers by one or more intervening layers.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may likewise be embodied as a computer-readable medium claim, or in other forms, such as being embodied in a means-plus-function claim.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

The techniques introduced here can be embodied as special-purpose hardware (e.g., circuitry), as programmable circuitry appropriately programmed with software and/or firmware, or as a combination of special-purpose and programmable circuitry. Hence, embodiments may include a machine-readable medium having stored thereon instructions which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), magneto-optical disks, ROMs, random access memories (RAMs), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing electronic instructions. The machine-readable medium includes non-transitory medium, where non-transitory excludes propagation signals. For example, a processor can be connected to a non-transitory computer-readable medium that stores instructions for executing instructions by the processor.

Sensors such as gas, liquid or particulate sensors may suffer from drift, i.e. the signal produced may change over time without any change in the environment (e.g. composition of the environment or changes in pressure or temperature). This may limit the lifetime of the sensor and/or accuracy of its measurements. The present inventors have found that changes arising for such drift may be compensated for using gas sensor apparatus and methods as described herein.

FIG. 1 illustrates gas sensor apparatus 100 according to some embodiments of the present disclosure. Although the apparatus is described herein with reference to gas sensor apparatus, it will be understood that the apparatus may be, e.g. for use in detection of a target material in a liquid. Although the apparatus is described herein with reference to apparatus containing a gas sensor configured to detect a target gas, it will be understood that the target material may be a liquid target material or a particulate material contained within a fluid and the sensor may be selected accordingly.

The apparatus has a first gas flow path 110 between a first valve 115 and a gas sensor 150 and a second gas flow path 120 between a second valve 125 and the gas sensor 150. FIG. 1 illustrates apparatus having a valve assembly of a first and second valve, e.g. a first and second two-way valve. In other embodiments, the valve assembly may comprise or consist of a single three way valve may be used to direct flow of gas from the atmosphere to the first or second gas flow path.

It will be understood that a “valve” as used herein means any apparatus configured to allow or block gas flow through the valve and may be manually operable (e.g. by way of a manually operable tap) and/or may be electromechanical, e.g. a solenoid valve, controllable by a controller, e.g. a programmable controller.

With reference to FIG. 2, in use of the apparatus gas from an atmosphere is drawn into the apparatus by any method known to the skilled person, e.g. by use of one or more pumps (not shown). The responses of the gas sensor 150 are measured in a first state of the apparatus in which gas reaches the gas sensor from the first flow path 110 only (e.g. while the first valve 115 is open and the second valve 125 is closed) to give a first gas sensor measurement and for gas reaching the gas sensor in a second state of the apparatus in which gas reaches the gas sensor from the second flow path 120 only (e.g. while the second valve 125 is open and the first valve 115 is closed) to give a second gas sensor measurement. The gas sensor may be sealed from gas communication with any gas other than that from the first or second gas flow paths. The apparatus state may be alternated between the first and second states.

In some embodiments, the apparatus may be configured to exhaust gas out of the apparatus after exposure of the gas sensor 150 to gas from one of the first and second gas flow paths and before exposure to gas from the other of the first and second gas flow paths.

Gas flowing along the second gas flow path 120 may reach the gas sensor 150 substantially unchanged from its composition in the environment from which it was drawn.

Gas flowing along the first gas flow path 110 may pass through or over a filter material in a filter region or stage of the first gas flow path 110 between the first valve 115 and the gas sensor 150. The filter material may be disposed in a chamber 130 having an inlet and outlet in fluid communication with the first gas flow path. The chamber may be removable and/or may have a sealable opening, e.g. for replacement of the filter material therein.

The filter material may adsorb or absorb the target material. The filter material may react with the target material. In some embodiments, the filter material selectively removes only the target material from the gas it is exposed to. It will be understood that the filter material is selected according to the target material. In some embodiments, the filter material selectively removes the target material and one or more further materials from the gas it is exposed to. The filter material may be a desiccant, e.g. silica gel, which removes water in addition to the target material. The present inventors have found that silica gel may be used to withdraw both water and 1-MCP from an environment.

In some embodiments, the filter material is a molecular sieve. The average pore size of the molecular sieve may be selected according to the target material. In the case of a volatile organic compound, e.g. 1-MCP, the molecular sieve optionally has a pore size of at least 4 Å, optionally at least 5 Å or 10 Å.

In some embodiments, the filter material is active carbon.

Filtering of the target material has been described above with reference to a filter material. In other embodiments, a filter device, e.g. a mesh or HEPA filter for a particulate target material or a scrubber for a target compound, may be used.

The presence, concentration or a change in concentration of the target material may be determined by subtracting the first gas sensor measurement or a derivative thereof from the second gas sensor measurement or a derivative thereof. This determination may be made without needing a separate background reading.

The apparatus may be cycled between first and second states to allow for changes in the gas, such as changes in humidity, e.g. due to temperature or pressure changes, to be taken into account.

In some embodiments, the apparatus may be held in a second state for longer than the first state. Optionally, the apparatus is held in the second state for at least 60% or at least 70% of the time during which gas sensor measurements are made. The second state may be interspersed with periodic, and relatively short, changes to the first state to allow for a background measurement. In this way, near-continuous monitoring of a target material in an environment with accurate adjustments due to any changes in the background measurement (e.g. due to changes in the environment and/or the gas sensor) may be achieved.

In some embodiments, the apparatus may be held in a first state for longer than the second state, for example if the gas sensor degrades in the presence of the target material, e.g. due to poisoning of an active material of the gas sensor by the target material or if the gas sensor responds relatively rapidly when exposed to the target material but recovers to a baseline state relatively slowly after exposure. Optionally according to these embodiments, the apparatus is held in the second state for at least 80% or at least 90% of the time during which gas sensor measurements are made. By limiting exposure of the gas sensor to the target material, the lifetime of the gas sensor may be extended and/or accuracy of measurements may be maintained over a longer period.

The method described herein may allow for changes in responsiveness of the gas sensor over time to be taken into account, e.g. due to ageing of the gas sensor; changes in the composition of the atmosphere such as changes in humidity over time; and/or changes in environmental conditions such as changes in atmospheric temperature or pressure. In this way, environmental changes over time may be taken into account, e.g. over the course of a day, such as changes between morning, afternoon, evening and night or over a longer period such as seasonal changes over the course of a year.

With reference to FIG. 3, following admission of gas into one of the first and second gas flow paths, the apparatus may be placed in a closed state by closing it off from the external environment for a period of time, e.g. by closing both the first and second valves, before admission of gas into the other of the first and second gas flow paths. Closing off the apparatus from the external environment may be done if concentration of the target gas varies with time. The period of closed time may be selected according to a length of time the gas sensor takes to provide a signal with a sufficiently large signal-to-noise ratio in response to exposure to gas from the first or second flow path.

The apparatus may be cycled between the first state; a closed state following the first state; a second state; and a closed state following the second state. Gas in the apparatus may be removed therefrom, e.g. by a pump or by displacement with a gas that the gas sensor is not responsive to, between the first state and the second state.

The apparatus may be held in the first state, second state, or a closed state following the first or second state, for a predetermined period. The predetermined period may depend on, for example, an estimated concentration of the target material and/or responsiveness of the gas sensor to the target material.

By setting the apparatus to a closed state, the amount of target material that the gas sensor is exposed to and/or length of time that the gas sensor is exposed to the target material may be limited. This may extend lifetime of the gas sensor.

A measurement of a response of gas sensor 150 to gas from the first flow path is described herein as a “first gas sensor measurement” and a measurement of a response of gas sensor 150 to gas from the second flow path is described hereinafter as a “second gas sensor measurement”. It will be understood that the first and second gas sensor measurements may be taken in any order.

In some embodiments, the apparatus may be configured to direct gas from the first flow path and/or from the second flow path to a return loop (not shown) following exposure of the gas sensor to the gas for repeating measurement of the response of the gas sensor to the gas. Recycling the gas and repeating the measurement may occur while the apparatus is in a closed state in which the apparatus is closed off from the external environment, e.g. the first and second valves are closed.

The amount of the filter material and/or surface area of the filter material in fluid communication with gas flowing along the first gas flow path may be selected according to parameters including one or more of: the rate at which the target gas is withdrawn from the environment; the flow rate of the gas; and the target material absorption capacity per unit mass of filter at the environmental conditions. In the case where the filter material is a desiccant, the amount and/or surface area of the filter material may be selected, at least in part, according to the humidity of the environment.

With reference to FIG. 4, in the case where the filter material is a desiccant, a humidification stage, e.g. comprising or consisting of a water reservoir 140, may be disposed in the first gas flow path between the filter region and the gas sensor 150. Gas flowing along the first gas flow path may pass through or over water disposed in the water reservoir. The water content of desiccated gas flowing along the first gas flow path may thereby be returned to a level which is the same as or similar to water content of the gas in the environment from which it was drawn. The water disposed in the reservoir may have a salt dissolved therein. The water may be a saturated salt solution. The humidification stage may comprise a gel humidifier. The gel may or may not contain a salt. The humidity of the gas may be set to a predetermined level or range as described in, for example, L. B.

Rockland, Anal. Chem. 1960, 32, 10, 1375. “Saturated Salt Solutions for Static Control of Relative Humidity between 5° and 40° C.” or L. Greenspan, Journal of Research of the National Bureau of Standards-A. Physics and Chemistry Vol. 81 A, No. 1, 1977. “Humidity Fixed Points of Binary Saturated Aqueous Solutions”, the contents of which are incorporated herein by reference. The predetermined humidity level or range may be selected to maximise lifetime of the gas sensor. For example, the predetermined humidity level or range may be below a threshold humidity at which condensation forms on the gas sensor. A preferred humidity of a gas reaching the gas sensor is in the range of 60-95% in the case where the sensor is an organic thin film transistor and the target material is 1-methylcyclopropene.

If the gas sensor is not sensitive to water then the water source may be omitted from the apparatus.

FIG. 5 illustrates gas sensor apparatus according to some embodiments of the present disclosure. The apparatus is as described with reference to FIG. 4 except that the water source 140 is disposed such that all gas flowing through the apparatus comes into fluid communication with water from the water source before reaching the gas sensor 150. By exposing gas from both the first and second gas flow paths to water from a common water reservoir 140, gas reaching gas sensor 150 from both gas flow paths may have the same water content. The water content of the gas reaching the gas sensor may be controlled and/or set to a predetermined value as described with reference to FIG. 4. Any response of the gas sensor 150 to water vapour may consequently be eliminated when the second gas sensor measurement is subtracted from the first gas sensor measurement.

FIGS. 1, 4 and 5 illustrate gas sensor apparatus in which gas flowing along the second gas flow path reaches the gas sensor 150 substantially unchanged from gas in the environment.

In other embodiments, the filter material disposed in the first gas flow path may selectively remove the target material and one or more further materials from the gas it is exposed to, and a further, different, filter material for selectively removing only the one or more further materials from the gas may be disposed in the second gas flow path. For example, a desiccant filter material for withdrawing water and the target material may be disposed in the first gas flow path and a further desiccant material for withdrawing only water from the target material may be disposed in the second gas flow path. In some embodiments, the gases may reach the gas sensor in a desiccated state. Desiccation of gases reaching the gas sensor may eliminate any response of the gas sensor due to water.

In some embodiments, two or more different filter materials are disposed in the first gas flow path wherein the different filter materials have different rates of absorption and/or different absorption capacities (measured as grams of target material that the filter material can absorb per gram of filter material) for a material in the gas drawn from the environment. In some embodiments, a first filter material may absorb the target material at a faster rate and/or to a greater capacity than a second filter material and the second filter material may absorb a non-target material (e.g. water) at a faster rate and/or to a greater capacity than the first filter material.

In some embodiments, a material other than the target material, e.g. water, may be removed by a suitable filter material before gas drawn from the environment reaches the valve arrangement. In some embodiments, a desiccant is disposed at the entrance to, or in, the gas inlet. In some embodiments, desiccated gases may be rehydrated before reaching the gas sensor, e.g. by exposure to a water reservoir as shown in FIG. 4.

FIG. 6 illustrates a gas sensor apparatus as illustrated in FIG. 5 and further comprising a dehumidification stage 160. The dehumidification stage may comprise a chamber comprising a desiccant disposed between or at the gas inlet and the valve assembly. Accordingly, an input gas is dehumidified by the desiccant in dehumidification stage 160 and subsequently rehumidified by water in reservoir 140. The extent of rehumidification may be controlled by, without limitation, a saturated salt solution or gel humidifier as described with reference to FIG. 4.

The present inventors have found that dehumidification of an input gas from an atmosphere followed by rehumidification of the dehumidified gas may result in less variation in humidity of the rehumidified output gas (arising from, e.g., variations in humidity of the input gas) than an output gas which has been subjected to only one of dehumidification and humidification.

The desiccant disposed in chamber 160 may selectively remove water only from the gas. In some embodiments, the desiccant is a molecular sieve which may have a size selected to remove water but not a target material, e.g. a molecular sieve having a pore size of less than 4 Å, e.g. 3 Å if the target gas is an organic compound such as an alkene, e.g. 1-MCP or ethylene; an alcohol, e.g. ethanol; or CO2. The desiccant may be a solid state salt. The salt may be an ammonium, alkali, alkali earth or transition metal salt. The salt may be, without limitation, a halide, hydroxide, sulfate, acetate, dichromate, formate or nitrate. Exemplary salts include, without limitation, NH₄NO₃, (NH₄)₂SO₄, LiCl, NaCl, MgCl₂, KCl, KOH, KBr, KI, NaBr, Mg(NO₃)₂, NaNO₃, KNO₃, sodium or potassium acetate, sodium or potassium dichromate, calcium formate and copper sulfate.

FIG. 7 illustrates a gas sensor apparatus as described with reference to FIG. 6 except that the dehumidification stage 160 is disposed in the second gas flow path 120, i.e. between the valve assembly and the gas sensor 150. Gas flowing in the first gas flow path 110 may or may not be dehumidified by the filter material disposed in the first gas flow path.

It will be appreciated that the filter material for removal of the target material according to these embodiments may or may not also remove water. It will be appreciated that dehumidification and rehumidification of an input gas to control humidity of an output gas may be used in arrangements other than those having first and second gas flow paths as described in FIGS. 1-7.

FIG. 8 illustrates gas sensor apparatus according to some embodiments of the present disclosure. The apparatus comprises a dehumidification stage 160 for dehumidification of a gas drawn into the apparatus, a gas sensor 150 and a rehumidification stage 140 disposed in a fluid flow path between the dehumidification stage and the gas sensor 150. The apparatus may contain a single gas flow path between an inlet of the apparatus and the gas sensor, as illustrated in FIG. 8. The apparatus may contain two or more gas flow paths between an inlet of the apparatus and the gas sensor. Dehumidification and rehumidification may be achieved as described anywhere herein, e.g. with respect to FIGS. 4, 5, 6 and 7.

FIGS. 1, 4, 5, 6 and 7 illustrate gas sensor apparatus having a valve assembly comprising two-way valves. In other embodiments, the valve assembly may comprise or consist of a 3-way valve.

FIGS. 1, 4, 5, 6 and 7 illustrate gas sensor apparatus having first and second gas flow paths between a valve arrangement and a gas sensor. In other embodiments, the gas sensor apparatus may have one or more further gas flow paths between the valve arrangement and the gas sensor, e.g. a third gas flow path between a third valve and the gas sensor. In some embodiments, the second gas flow path may not contain any filter material for withdrawing any material in the gas; and the first and third gas flow paths may contain a material for removal of different materials or different combinations of materials from the gas. This may be particularly advantageous if the gas sensor is responsive to both the target material and one or more other materials in the environment, e.g. water.

If the gas sensor is responsive to both the target material and another material in the gas, e.g. water, then the first gas flow path may contain a material for withdrawing both of these materials from the gas and the third gas flow path may contain a material for withdrawing only one of these materials from the gas. The responses of the gas sensor to gases from the first and third gas flow paths may be used to determine the response due to the target material and the response due to the other material that the gas sensor is responsive to. If the gas sensor is responsive to water, this arrangement may eliminate the need to rehydrate gas reaching the gas sensor if a filter material which withdraws both the target material and water from the gas is used.

Subtraction of the first gas sensor measurement or a derivative thereof (in which any target material has been absorbed by the filter material) (from the second gas sensor measurement or a derivative thereof (in which the target material has not been removed from the gas reaching the gas sensor, e.g. in which the gas is substantially unchanged from its composition in the external environment), may provide a concentration value of the target material or a value from which the concentration may be derived.

A response of the gas sensor to known concentrations of the target material may be measured for derivation of the concentration value from the first and second gas sensor measurements.

The gas sensor apparatus may be in wired or wireless communication with a processor configured to receive measurements from the gas sensor. The processor may be configured to determine the presence, concentration and/or change in concentration of the target material from the received measurements.

The processor may be in wired or wireless communication with a display, user interface or controller. In the case where the target material is 1-MCP, the gas sensor apparatus may be configured to send a signal to the display, user interface or controller if 1-MCP concentration as determined from measurements of the gas sensor apparatus falls below a threshold concentration. The controller may be configured to activate a 1-MCP source for release of 1-MCP into the environment upon receiving a signal from the processor that 1-MCP concentration has fallen below a threshold concentration.

The sensors described herein may be gas, liquid or particulate sensors, preferably gas sensors, and may be selected from any sensor known to the skilled person including, without limitation, semiconductor sensors, e.g. semiconductor gas sensors; photoionisation detectors; electrochemical sensors and IR sensors, pellistors, optical particle monitors, quartz crystal microbalance sensors, surface acoustic wave sensors (SAWS), cavity ring-down spectroscopy (CRDS) sensors and biosensors. Semiconductor sensors may comprise an organic semiconductor, an inorganic semiconductor or a combination thereof. The organic semiconductor may be polymeric or non-polymeric.

Exemplary semiconductor sensors are thin film transistor sensors; vertical or horizontal chemiresistor sensors; and metal oxide semiconductor sensors.

Metal oxide and photoionisation detectors may function most effectively in a dry environment. Accordingly, in some embodiments gas drawn into apparatus comprising a metal oxide sensor or photoionisation detector may be dehumidified by at least one of a material which removes water but not the target material and a filter material which removes both of water and the filter material, but not rehumidified before reaching the gas sensor.

Electrochemical sensors may provide more stable signals and/or have longer lifetime if exposed non-continuously to a target gas as described herein.

The first and second gas sensor measurements may be selected according to the sensor type. In the case of a thin film transistor sensor, the measurement or a derivative thereof may be one or more of a change in drain current; a change in resistance; or a gradient of a change over time, e.g. dI/dt in which I is drain current and t is time.

FIGS. 9-12 describe various gas sensors. It will be appreciated that the sensors according to these embodiments may alternatively be used as liquid or particulate sensors.

FIG. 9 is a schematic illustration of a bottom contact bottom gate TFT gas sensor suitable for gas sensor apparatus as described herein. The bottom contact bottom gate TFT comprises a gate electrode 203 over a substrate 201; source and drain electrodes 207, 209; a dielectric layer 205 between the gate electrode and the source and drain electrodes; and an semiconductor layer 211 extending between the source and drain electrodes. The semiconductor layer 211 may at least partially or completely cover the source and drain electrodes.

FIG. 10 is a schematic illustration of a top-contact bottom gate TFT gas sensor suitable for gas sensor apparatus as described herein. The top-contact bottom gate TFT is as described with reference to FIG. 9 except that the semiconductor layer 211 is between the dielectric layer 205 and the source and drain electrodes 207, 209.

FIG. 11 is a schematic illustration of a top gate, bottom contact TFT gas sensor suitable for a gas sensor system as described herein. The top gate bottom contact TFT comprises source and drain electrodes 207, 209 on substrate 201; a semiconductor layer 211; and a dielectric layer 205 between the gate electrode 203 and the semiconductor layer.

FIG. 12 is a schematic illustration of a top gate, top contact TFT gas sensor suitable for gas sensor apparatus as described herein. The top gate, top contact TFT is as described with reference to FIG. 11 except that the semiconducting layer 211 is between the substrate 201 and the source and drain electrodes 207, 209.

The dielectric layer of a top-gate TFT gas sensor as described herein may be a gas-permeable material, optionally an organic material, which allows permeation of the gas to be sensed through the dielectric layer to the semiconducting layer. The top-gate top-contact TFT is as described with reference to FIG. 11 except that the semiconductor layer 211 is between the dielectric layer 205 and the source and drain electrodes 207, 209.

The gate electrode of a top-gate TFT gas sensor as described herein may be a gas-permeable material, optionally an organic conducting material, and/or may be a patterned electrode defining gaps through which gas may pass, e.g. a conductive strip having fingers extending therefrom with a gap between adjacent fingers or a patterned conductor having apertures formed therein.

The target material may be a volatile organic compound. It will be understood that the target material may be an evaporated organic compound which may have a boiling point at the pressure of the environment, which is higher than the temperature of the environment, e.g. an organic compound having a boiling point above 25° C. at 1 atmosphere.

In some embodiments, the target material is an alkene. The alkene may be an acyclic alkene, e.g. ethylene. The alkene may comprise an alkene group substituted with an aromatic group, e.g. styrene. The alkene may be cyclic, e.g. 1-MCP.

In some embodiments, the target material is an alkane, e.g. methane.

In some embodiments, the target material is an ester. The ester may be a C₁₋₁₀ alkyl ester or C₁₋₁₀ alkanoate ester, optionally C₁₋₁₀ alkyl-C₁₋₁₀ alkanoate esters, e.g. ethyl acetate; ethyl butanoate; ethyl hexanoate; propyl acetate; butyl acetate; butyl butanoate; butyl hexanoate; pentyl acetate; hexyl acetate; hexyl butanoate; hexyl hexanoate; 2-methylpropyl acetate; 2-methylbutyl acetate; ethyl 2-methylbutanoate; butyl 2-methylbutanoate; and hexyl 2-methylbutanoate.

In some embodiments, the target material is a polar compound. The target polar compound may be a hydrocarbon which does not have a mirror plane bisecting a carbon-carbon bond of the hydrocarbon. The target polar compound may have dipole moment of greater than 0.2 Debyes optionally greater than 0.3 or 0.4 Debyes. The target polar compound may be 1-MCP.

In some embodiments, the desiccant is silica gel and the target polar compound is 1-MCP.

EXAMPLES

OTFT Gas Sensor

A PEN substrate was baked in a vacuum oven and then UV-ozone treated for 30 seconds. Source and drain contacts were deposited onto the substrate by thermal evaporation of 3 nm Cr followed by 40 nm Au through shadow masks with channel length of 125 μm and a channel width of 4 mm. Semiconducting Polymer 1, illustrated below, was deposited over the substrate by spin coating from a 1% w/v solution in 1,2,4-trimethylbenzene to a thickness of 40 nm and dried at 100° C. for 1 minute in air. The polymer dielectric Teflon® AF2400 was spin coated from a 2.5% w/v solution in a 50:50 v/v blend of fluorinated solvents FC43 and FC85 to a 300 nm thickness and dried at 80° C. for 10 min, after a 5 minute initial drying phase while spinning. The gate was formed by thermal evaporation of Cr (3 nm) followed by Al (200 nm) through a shadow mask to form a gate electrode having a comb structure with comb fingers of 125 microns width and gaps of 125 microns between fingers.

Gas Sensor Measurements

The OTFT Gas Sensor was provided in an apparatus as described in FIG. 5 containing a 40 ml or 100 ml vial filled with silica gel pellets and a 20 ml vial containing water for gas to flow over. 1-MCP was introduced into the apparatus by adding cyclodextrin containing a known amount of 1-MCP (4.3 wt %) to water. It was assumed that all 1-MCP was released by this process to give an assumed (theoretical) concentration of 1-MCP.

Measurements were made according to steps (i)-(iv):

-   (i) The OTFT Gas Sensor was exposed for 1 hour to air from the first     gas flow path (i.e. with 1-MCP removed by the desiccant) and a     change in resistance was determined, from drain current at     Vd=Vg=−4V, to provide a baseline. -   (ii) The OTFT Gas Sensor was then exposed for 20 minutes to air     passing through the second gas flow path with 1-MCP present at a     theoretical concentration of 0.5 ppm. -   (iii) The OTFT was allowed to recover by exposure to air from the     first gas flow path for 2 hours. -   (v) Steps (ii) and (iii) were repeated twice, in which the     theoretical concentrations of 1-MCP in the second and third     iterations were 1 ppm and 4.5 ppm respectively.

The change in resistance of the OTFT Gas Sensor was measured periodically throughout steps (ii)-(iv) and the background measurement of (i) and (iii) was subtracted.

With reference to FIGS. 13A and 13B, the change in resistance, following background subtraction, shows a linear relationship with the theoretical 1-MCP concentration.

The response of the OTFT Gas Sensor was measured as described above at different 1-MCP concentrations except that exposure in step (ii) was for 5 minutes rather than 20 minutes.

With reference to FIG. 14, a stronger response (larger signal to noise ratio) is achieved for longer exposure of the OTFT Gas Sensor to 1-MCP.

Rate of Change Measurement

The sensitivity of a gas sensor to a target material can vary between sensors made at different times, e.g. made in different batches which may affect the accuracy of measurement.

Multiple OTFT Gas Sensors prepared as described above, but in different batches, were exposed to increasing concentrations of 1-MCP with recovery periods of no 1-MCP exposure between each exposure.

With reference to FIG. 15A, there is a significant variation in measured drain current at Vds=Vg=−4V. As shown in FIG. 15B, this variation results in relatively large error bars which increase as the magnitude of the response increases.

The data from these multiple devices as rate of change of drain current (dI/dt) vs time shows a much more uniform response across the devices as compared to drain current vs. time, giving a linear correlation with 1-MCP concentration with much smaller error bars as shown in FIGS. 16A and 16B,

A rate of change of the gas sensor response to a change in target material concentration can be considerably more significant than other factors causing a change in the gas sensor response such as changes in the background environment and/or ageing of the gas sensor.

Consequently, a relatively large change in the rate of change of response of the gas sensor may be attributed to a change in concentration of a target material. By identifying the start of this relatively large change, and subtracting it from the rate of change prior to the start of this large change event, it may be unnecessary to separately measure a response of the gas sensor to a background environment and/or provide a control gas sensor.

The gas sensor apparatus and method as described herein may be used in monitoring the concentration of a gas in an environment, e.g. a concentration of 1-MCP in a location where harvested fruit or plants are stored, such as a warehouse or store, or concentration of 1-MCP during transportation of harvested fruit or plants.

Humidity Control

Air having varying levels of humidity (input air) was treated by either humidification only or dehumidification followed by rehumidification to produce an output air.

Input air having a humidity of about 12% or about 90% was passed at a fixed flow rate of 50 cm³/min and temperature of 5° C. over a 30 ml vial containing dry LiCl salt for dehumidification and subsequently over a 40 ml vial containing about 30 ml of saturated NaCl solution for rehumidification. For comparison, input air was treated in the same way but without dehumidification.

With reference to FIG. 17A, changing input air from about 12% humidity to about 90% humidity is accompanied by an increase of about 10% in humidity of the output air.

With reference to FIG. 17B, changing input air from about 12% humidity to about 90% humidity is accompanied by a much smaller change in humidity (about 3%) of the output air. The gas sensor apparatus and method as described herein may be used in monitoring the concentration of a target material in an environment in which the target material concentration is allowed to change naturally in the environment or in which the target material is artificially introduced or removed from the environment, e.g. introduction of 1-MCP into an environment, which may result in an irregular change in concentration of the gas over time. 

1. A sensor system configured to determine a presence, a concentration or a change in concentration of a target material in a gaseous or liquid environment, comprising: a sensor configured to respond to the target material; an inlet configured to draw gas from an environment into the apparatus; and a valve arrangement configured to direct fluid drawn from the environment to a first fluid flow path in fluid communication with the sensor or to a second fluid flow path in fluid communication with the sensor.
 2. The system according to claim 1, wherein the first fluid flow path is disposed between the sensor and a first valve of the valve assembly and the second fluid flow path is disposed between the sensor and a second valve of the valve assembly.
 3. The system according to claim 1 wherein the first and second fluid flow paths are disposed between the sensor and a three-way valve of the valve assembly.
 4. The system according to claim 1, wherein the system further comprises a third fluid flow path in fluid communication with the sensor and disposed between a valve of the valve arrangement and the sensor.
 5. The system according to claim 1, wherein a water reservoir in fluid communication with the first fluid flow path is disposed in fluid communication with the first fluid flow path between the valve assembly and the sensor.
 6. The system according to claim 5, wherein the water reservoir is in fluid communication with the second fluid flow path.
 7. The system according to claim 6, wherein a saturated salt solution is disposed in the water reservoir.
 8. The system according to claim 6 wherein a desiccant is disposed between the inlet and the valve assembly.
 9. The system according to claim 6 wherein a desiccant is disposed in the second fluid flow path.
 10. The system according to claim 1, wherein the sensor is a thin film transistor.
 11. The system according to claim 1, wherein a filter material for removing the target material from the fluid is disposed in the first fluid flow path.
 12. The system according to claim 11, wherein the filter material is a desiccant filter material.
 13. The system according to claim 11, wherein the filter material comprises silica gel.
 14. The system according to claim 11, wherein the filter material is a molecular sieve.
 15. The system according to claim 1 wherein the system is a gas sensor system and the sensor is a gas sensor.
 16. A kit for forming the sensor system according to claim 1, comprising a sensor, a fluid inlet, a first fluid flow path, a second fluid flow path and at least one valve for forming the valve arrangement.
 17. (canceled)
 18. A method of determining a presence, concentration or change in concentration of a target material in a gaseous or liquid environment, the method comprising: measuring, in either order, a first response and a second response of a sensor of a sensor system wherein the sensor is configured to respond to the target material and wherein the sensor system comprises: the sensor; an inlet configured to draw gas from an environment into the apparatus; and a valve arrangement configured to direct fluid drawn from the environment to a first fluid flow path in fluid communication with the sensor or to a second fluid flow path in fluid communication with the sensor, wherein the first response is a response of the sensor to fluid from the first flow path which has been treated to remove the target material from the fluid and the second response is a response of the sensor to fluid from the second flow path which has not been treated to remove the target material from the fluid; and subtracting the first sensor measurement or a derivative thereof from the second sensor measurement or a derivative thereof.
 19. The method according to claim 18, wherein a filter material for removing the target material from the fluid is disposed in the first fluid flow path.
 20. The method according to claim 18, wherein the first sensor measurement is generated by exposing the sensor to a sample from the fluid environment which has been exposed to a desiccant filter material and rehydrated.
 21. (canceled)
 22. The method according to claim 18, wherein the target material is 1-methylcyclopropene. 23-34. (canceled) 